1. Field of the Invention
The present invention relates to an image pickup apparatus which uses an image pickup device such as a video camera, and more particularly to an exposure control therefor.
2. Related Background Art
Iris vanes, for directly controlling the amount of light incident onto an image pickup device, have commonly been used as means for attaining a function called the exposure correction in video cameras. However, in the iris vane approach, iris vanes and peripheral mechanical components are required and have been a barrier in attaining a small size, small weight and low cost device. As one of means for solving the problem, a method for controlling the exposure by an electronic shutter function which controls a charge accumulation time of the image pickup device, without using iris vanes, has been proposed.
However, in the exposure control which uses the iris vanes, the iris vanes are in a fully closed position when a power of the image pickup apparatus is turned off so that the incidence of a light to the image pickup device is prevented. On the other hand, when the iris vanes are removed and the exposure control is effected only by the electronic shutter function, the light of the object remains directed on the image pickup device even after the power of the image pickup apparatus is turned off. As a result, if a light from an extremely high brightness object is directed on the image pickup device when the power of the image pickup apparatus is turned on, a large amount of charges are generated in the image pickup device and the charges overflow from pixels and flow into adjacent pixels or vertical transfer areas (blooming phenomenon and smear phenomenon), resulting in a very dirty screen.